1. Field of the Invention
The present invention relates to a magneto-resistive effect (MR) element and particularly to a configuration of a spacer layer.
2. Description of the Related Art
Reproducing heads with high sensitivity and high output are in demand in conjunction with condensing of high recording density in hard disk drives (HDD). As an example of this type of reproducing head, a spin valve head has been developed. A spin valve head includes a nonmagnetic metal layer and a pair of ferromagnetic layers positioned on both sides of the nonmagnetic metal layer in a manner of contacting the nonmagnetic metal layer. The magnetization direction of one of the ferromagnetic layers is pinned in one direction (hereinafter, this type of layer is referred to as a magnetization pinned layer), and the magnetization direction of the other freely rotates in response to an external magnetic field (hereinafter, this type of layer is referred to as a magnetization free layer). When an external magnetic field is applied, the relative angle of the spins between the magnetization pinned layer and the magnetization free layer changes so that magneto-resistive change is realized. Typically, the magnetization direction of the magnetization pinned layer is pinned by utilizing the exchange coupling force of an anti-ferromagnetic layer.
On the other hand, in order to realize further condensing of high recording density, reducing a read gap (a distance between upper and lower shield layers) is required. However, when the read gap is reduced to approximately 20 nm, it is difficult to place an anti-ferromagnetic layer within the read gap. Therefore, a configuration has been developed in which a pair of magnetization free layers is arranged on both sides of a spacer layer. According to this configuration, reduction of the read gap is easily realized because no anti-ferromagnetic layer is required.
In either configuration, the spacer layer is a necessary component to realize a magneto-resistive change, and promising spacer layer materials have been developed to achieve a large magnetoresistance ratio (hereinafter referred to as MR ratio). Conventionally, a technology has been known that an MR ratio is increased by disposing a resistance adjustment layer in a spacer layer and narrowing a path where a sense current flows. However, because a sense current is concentrated to a conductive part so that a current density is increased, this technology is not preferable from the view point of reliability. U.S. Patent Application Publication No. 2008/0062557 discloses a technology that an oxide semiconductor layer such as ZnO, TiO or the like is used as a part of a spacer layer. Because this technology allows to prevent a sense current from being concentrated, it is possible to increase the reliability as increasing the MR ratio.
A ferromagnetic layer adjacent to the spacer layer is normally composed of Co, Ni, Fe or the like as a primary component. When these elements are positioned in a manner of contacting the oxide semiconductor layer, the ferromagnetic layer is oxidized due to an oxidization action of oxygen contained in the oxide semiconductor layer, polarizability is decreased, and therefore there is a possibility for the MR ratio to fall. U.S. Patent Application Publication No. 2008/0062557 discloses a technology that a nonmagnetic metal layer composed of copper, gold, silver or the like is disposed between a ferromagnetic layer composed of CoFe or the like and an oxide semiconductor layer. It is expected that these nonmagnetic metal layers prevent expansion of oxygen contained in the oxide semiconductor and oxidization of the ferromagnetic layer; however, a certain film thickness is required to function as an antioxidant film. On the other hand, when the film thickness is too large, an antioxidant effect can be obtained but electrons are more likely to be scattered, so that the MR ratio is decreased. For these reasons, it is difficult to maintain a balance between two demands that are the antioxidant and the prevention of electron scattering.
It is an object of the present invention to provide an MR element with which oxidization of a magnetic layer adjacent to a spacer layer is prevented and a large MR ratio is realized.